In the following experiment we will synthesize a racemic mixture of the Λ and Δ forms of Co(en)33+. The product mixture contains eight isomers: four conformers with a Λ absolute configuration and four conformers with a Δ absolute configuration. The pairs listed below are enantiomers of each other, and since each pair has the same energy, we will base all the following arguments using only the Λ isomers.
The conformers are all very similar in energy and depending on the chemical environment, one or more of the conformers will be slightly more stable. Originally it was assumed that the Λ-δδδ was the most stable conformer as it was the most common form found in X-ray structure determinations. All of the other conformers were observed however when the anions were changed. The extent of hydrogen bonding and the crystal packing forces were postulated to determine what conformers were found. So for example, The phospahate anion has been seen to interact with all three hydrogens on a single face of the Λ-δδδ isomer whereas previously it was believed that hydrogen bonding favored the more open form (the Λ-λλλ) and this is indeed the case when the anions take up more physical space.
The relative energies of the conformers can be determined computationally. We will either give you the results of our computations in a simulated water solvent, or will collectively do the computations over the course of our experiment
The four Λ conformers are shown below:
|
[Λ-δδδ-Co(en3)]3+ |
[Λ-δδλ-Co(en3)]3+ |
[Λ-δλλ-Co(en3)]3+ |
[Λ-λλλ-Co(en3)]3+ |
You will need to familiarize yourself with the nomenclature and symmetry of these molecules. To do so,
Go to our website on the The Structure and Symmetry of Metal Tris Chelates and
study the
assignment of absolute configuration (Section C) and λ and δ twists in ligand chelate rings (Section F).